In electrohydraulic systems which include a plurality of electrohydraulic devices, such as servo actuators, motors and pumps, it is conventional practice to couple all of such devices to a remote master controller for coordinating or orchestrating device operation to perform a desired task. Motors and actuators may be employed, for example, at several coordinated stages of a machine tool line for automated transfer and machining of parts at a series of workstations. In accordance with conventional practice, the master controller may comprise a programmable controller or the like coupled through individual digital-to-analog converters to the various remotely-positioned electrohydraulic devices for supplying control signals thereto. For closed-loop operation, a sensor is positioned at each electrohydraulic device for sensing operation thereof, and feeds a corresponding signal to the master controller through an analog-to-digital converter.
Thus, in a system which embodies a plurality of electrohydraulic devices, a substantial quantity of electrical conductors must be provided for feeding individual control signals to the various devices and returning sensor signals to the master controller. Such conductors interfere with system design and operation, and are subject to failure. The bank of d/a and a/d converters for feeding signals from and to the master controller add to the expense and complexity of the overall system. Perhaps most importantly, system performance is limited by capabilities of the master controller. For example, a programmable controller may require one hundred milliseconds to scan a device sensor signal, compute a new control signal and transmit such control signal to the remote device. Such overburdened programmable controller operations are not acceptable in high performance applications which may require a six millisecond response time, for example, at each of a plurality of remote devices.
It is therefore a general object of the present invention to provide an electrohydraulic servo system which exhibits the fast response time necessary for high performance applications, while at the same time reducing cost and complexity which inhere in prior art system of the character described above. In furtherance of the foregoing, a more specific object of the invention is to provide a system of the described character wherein each of the system electrohydraulic devices embodies microprocessor-based control adapted to communicate with a central or master controller and for thereby distributing control of the several electrohydraulic devices while maintaining overall coordination thereamong.
Electrohydraulic servo valves are conventionally employed for controlling operation of hydraulic devices, such as rotary actuators, linear actuators and hydraulic motors for example. Such servo valves are conventionally controlled by remotely positioned master electronics as described hereinabove, whether operating individually or as part of a coordinated system. A further object of the present invention, therefore, is to provide an electrohydraulic servo valve assembly which embodies on-board microprocessor-based control electronics. In furtherance of the foregoing, as well as the system objectives previously set forth above, yet another object of the invention is to provide an electrohydraulic servo valve assembly which includes facility for connection to the sensor on the device with which the servo valve is associated for facilitating local closed-loop servo control of the same, while at the same time embodying facility for communication with a remote master controller to obtain coordinated operation with other system devices.
In systems which embody a servo valve coupled to a hydraulic actuator, particularly a linear actuator, it is conventional practice to monitor actuator position using an electroacoustic linear displacement transducer marketed by Temposonics, Inc. of Plainview, N.Y., and disclosed in U.S. Pat. No. 3,898,555. This transducer includes a magnet coupled to the actuator piston for motion conjointly therewith, and a electroacoustic waveguide adjacent to the path of the magnet. A current pulse is launched on a wire which extends through the waveguide and coacts with the magnet to launch an acoustic signal within the waveguide. A coupler or mode converter receives such acoustic signal, with the time between the launching of the current pulse and receipt of the acoustic signal being a function of position of the magnetic relative to the waveguide. This transducer is durable, is directly mounted on the actuator cylinder but magnetically rather than physically coupled to the actuator piston, and is capable of providing an accurate indication of actuator piston position. However, conventional electronics for obtaining such position readings are overly complex and inordinately expensive. Furthermore, such electronics are conventionally supplied in a separate package which must be appropriately positioned and protected in the actuator operating environment. Another object of the present invention, therefore, is to provide inexpensive electronics for coupling to actuator position transducers of the described character. In furtherance of the objectives set forth above relative to provision of a servo valve assembly with on-board control electronics, another object of the present invention is to provide transducer interface electronics of the described character which are sufficiently compact for inclusion in such servo valve on-board control electronics package.
Another problem in the art of electrohydraulic servo valve control lies in overcoming effects of temperature on the valve coil. Coil force is proportional to current. Valve coils are conventionally driven by constant current amplifiers so that change in coil resistance due to temperature has little affect. However, such constant current amplifiers are bulky and expensive. Constant voltage amplifiers are preferable in terms of size and expense, but control of current and force becomes a problem. Another object of the present invention is to provide a valve coil arrangement with reduced temperature sensitivity, and which can thus be used with constant voltage amplifiers of the type described. A further object of the invention is to provide improved valve driver electronics characterized by reduced cost, reduced generation of electromagnetic interference, and/or increased safety at the load.